JC31: 'Ingredients' of the Southern Ocean


Cruise diary

Tuesday 24 February 2009
Location: Drake Passage (55.83°S/57.82ºW)

Intertial currents in the Southern Ocean

Since we've been in Drake Passage, I've been on the hunt for inertial currents in the upper ocean currents measured with the RRS James Cook's shipboard acoustic doppler current profiler (ADCP). The Southern Ocean is a pretty good place to look for the inertial currents because it is very windy here with lots of storms blowing through. As a swing swings or a bell rings when pushed and let go, the ocean 'rings' with inertial currents when the surface winds change in time. These inertial currents rotate at a rate that depends on the latitude and are bigger at the surface than below it (Fig. 1).

However, the inertial currents aren't the only currents in the upper ocean, larger geostrophic currents driven by lateral differences in temperature and salinity typically dominate any oceanic measurements of currents. Thus, picking out the inertial currents from other flows is a bit like listening for the gentle chiming of a bell while the the rest of the orchestra is going at full blast. The reason looking for these inertial currents is worth the trouble is that they tell us how far down the wind energy is transmitted into the ocean, and hopefully will also tell us how much wind energy is available for mixing heat and salt between the different oceanic layers.

The two things in my favor then are that (1) inertial currents decay much more quickly with depth than geostrophic currents; and (2) the inertial currents rotate in time at the inertial frequency, much faster than geostrophic currents change. One remaining consideration then is to look for inertial currents in measurements taken over a period of time at a single location. If we are not careful to exclude data taken while we are moving along, then we can not be certain that the variation in currents is not due to changes in spatial patterns instead of changes in time. A preliminary analysis of upper ocean currents measured while we were standing by for repairs and the deployment of a hydrographic cast, shows that there is enhanced shear (i.e. vertical changes in the horizontal currents, see image on left) between 50 and 100-m, corresponding to an interface between oceanic layers.

If we are to look at the direction of the sheared currents at these depths, we see the angles gradually increase with time (see image, right), indicating that there is anticlockwise rotation, which is what we expect of inertial currents in the Southern Hemisphere. All of which is really promising!

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